The invention concerns the examination of a solidifying and/or hardening material, such as cement, concrete or the like, using ultrasound waves, emitted from an ultrasound transmitter to an ultrasound receiver, which penetrate the material and are continuously measured and analyzed.
Such examinations are known e.g. from the publication xe2x80x9cKontinuierliche Ultraschallmessungen wxc3xa4hrend des Erstarrens and Aushxc3xa4rtens von Betonxe2x80x9d (continuous ultrasound measurements during solidification and hardening of concrete) by Chr. U. Grosse and H.-W-Reinhardt in Otto-Graf-Journal, Vol. 5, 1994.
Ultrasound waves can penetrate a material without causing damage thereby being influenced by the elastic properties of the material, which produces information about the elastic properties.
With concrete, these are e.g. its current solidification and hardening state, composition (grading curve, water-cement value etc.) and the entrained air content and possibly utilized additional means.
In industrial construction e.g., determination of solidification start and end of cement paste according to DIN EN 196, part 3, is carried out through the Vicat method. A measurement of this type is not possible with concrete due to the aggregate and is therefore not provided in the above-mentioned standard. Examination methods for unset concrete have been, on the one hand, consistency measuring methods such as the propagation test and compacting test according to DIN 1048 part 1, the penetrometer according to ASTM C-403 and the setting test according to DIN ISO 4109. On the other hand, there is the air content measurement according to DIN 1048 part 1 including pressure compensation method and furthermore methods for determining the water content.
The latter methods permit only individual measurements at fixed points in time and give information about a certain property. It is not possible to obtain detailed information about the composition of the material nor about the further hardening of the material after solidification.
It is therefore the underlying purpose of the invention to provide reliable use of an ultrasound test method in industrial practice and permit easy continuous monitoring of the state of a solidifying and/or hardening material.
This object is achieved by a method for examining a solidifying and/or hardening material such as cement, concrete or the like, using ultrasound waves emitted by an ultrasound transmitter, which penetrate the solidifying and/or hardening material, are continuously measured and analyzed, comprising the following method steps:
i) during solidification and/or hardening of the material, the signal shapes of the ultrasound waves penetrating the material are recorded;
ii) The change with time of the compression wave velocity and/or the relative energy of the ultrasound waves and/or the frequency spectra of the ultrasound waves is extracted from the ultrasound wave shapes during the entire course of solidification and/or hardening of the material.
iii) This change with time of the compression wave velocity and/or the relative energy o the ultrasound waves and/or the frequency spectra of the ultrasound waves is approximated through a compensating function, preferably the Boltzmann function.
iv) the free parameters of the compensation function are associated with material properties.
v) the free parameters of the compensation function permit comparison of a current measurement with reference values of these parameters to permit determination of material properties of the examined material.
Automatic measuring and analysis of the data is largely possible and information about the material itself can be obtained already during the solidifying/hardening phase.
For the measurement, the material to be examined is introduced into a receptacle and compacted. The opposing sides of the receptacle are provided with a preferably broad-band (i.e. adequately linear frequency response function over a broad spectral range) ultra sound transmitter and a corresponding receiver. Same transforms the acceleration signal into a voltage signal and transmits it to a computer-controlled analog-digital transformer card which stores the signal in digital form thereby making it accessible for further analysis.
For an analysis, the velocity of the compression wave vp(T), the relative energy E(T) of a measured signal, and the frequency spectrum f(T) of the signal can be extracted with corresponding algorithms. The velocity of the compression wave vp(T), the relative energy E(T) of a measured signal and the frequency spectrum f(T) of the signal depend on the time T elapsed since production of the material and form together a complete parameter set which contains the entire information about the material which can be obtained from elastic waves.
The wave velocity of the compression waves in the material can be determined from the quotient between running distance s and running time t(T) of the waves according to vp(T)=s/(t(T)xe2x88x92t0). While the running distance s, determined through the dimensions of the receptacle, is constant, the running time t(T) of the signals is reduced with increasing solidification of the material during the duration T of the test. In this calculation, constant parts for the running time of the waves through the walls of the container and for the time delay, caused by the measuring means, must be subtracted from the determined running time. This dead time t0 of the system which is not related to the material can be determined through calibration measurement, which can be achieved in the most simple fashion through running time measurement with direct coupling of transmitter and receiver container walls.
The relative energy E(T) is defined as a quotient of the wave energy which can be measured after passage of the wave through the material, and the energy which was introduced into the material by the ultrasound pulse. The individual energies are thereby calculated from the integral of the amplitude squares of the respective signals. If the introduced energy cannot be used as measuring value, it can be assumed to be constant when using a suitable ultrasound transmitter. The relative energy increases with increasing hardening or solidification of the material. The energy can further be represented as its integral overtime.
If the utilized ultrasound transmitter can generate sufficiently short impulses, the transmitted ultrasound wave contains more than one certain frequency. A broad continuous frequency spectrum up to a certain limit frequency is excited which is reciprocal to the impulse duration. Depending on the hardening or solidifying state, the material can transmit different frequency portions in a different manner. After the measurement, the spectrum of the signals can be calculated through Fourier transformation. If these individual spectra are normalized to their maximum, added chronologically and the spectral amplitudes are graphically represented as grey values, one obtains so-called contour plots. This three-dimensional representation permits calculation of frequency time curves or frequency time areas per individual measurement e.g. through calculation of average frequency maxima. These representations permit tracking of the spectral transition properties of the material as a function of time.
Correlation with previous measurements or with existing reference curves for velocity and energy produces e.g. findings concerning the composition of the material.
The measured curve shapes are examined more closely with respect to use of ultrasound technology within quality control with the aim of modelling the variation of the measured values (velocity, energy, frequency) with time in dependence on the material composition and nature. This is thus the solution of an inversion problem with unknown material properties. The inventive method facilitates classification of the material within quality control after adjustment to the respective task.
To achieve this object, functions with sufficient free parameters must be used by means of which the curve shapes which are typical for the change of the measured variable vp, E and f can be interpreted. The Boltzmann function which is known from thermodynamics is e.g. particularly suited for the velocity:       y    ⁢          xe2x80x83        ⁢          (      x      )        =                              A          1                -                  A          2                            1        +                  e                                    x              -                              x                0                                      dx                                +          A      2      
It contains the four free parameters A1, A2, x0 and dx whose values can be used for adjusting the compensating function to the measuring curves. The quality of the inversion curves calculated e.g. for the velocity is more than sufficient for the practical application of the method. All four free parameters can be used for a detailed classification of the materials. The parameter A2 e.g. can be associated with the water/cement value W/Z when examining unset concrete.
In a further development of the inventive method, an other embodiment is characterized in that the arrival time of an ultrasound wave (initial use) is determined automatically with an algorithm which is based on the sum of the partial energy of the digitized received signal, wherein the energy course Si of the digitized signal is determined by the sum of the amplitude squares xk2:       S    i    =            ∑              k        =        0            i        ⁢          xe2x80x83        ⁢          x      k      2      
wherein xk is the kth sample point of the digitized signal and the minimum of the energy course Si is determined which results from correction of Si with a trend xcex4:       S    i    xe2x80x2    =                    ∑                  k          =          0                i            ⁢              xe2x80x83            ⁢              x        k        2              -          i      ⁢              xe2x80x83            ⁢      δ      
with       δ    =                  S        N                    α        ·        N              ,
wherein SN is the partial energy at the last sample point N and xcex1 is iteratively determined through comparison of the corrected energy course Sixe2x80x2 with the measured wave shape of a received ultrasound signal, and the arrival time of the ultrasound wave (initial use) is associated with the minimum of the corrected energy course Sixe2x80x2.
In this embodiment, the arrival time of the ultrasound signal at the receiver is determined thereby producing the running time. To determine this so-called initial use, an algorithm has been developed which is based on the partial energy and the use of the Hinkley criterion, which permits a robust and very simple approach for initial use detection. The sum of the partial energy Si, of an individual digitized wave signal can be represented as sum of the amplitude squares xk2 as below:       S    i    =            ∑              k        =        0            i        ⁢          xe2x80x83        ⁢          x      k      2      
The sample point i thereby corresponds to a certain time during the signal. Arrival of the signal is thereby characterized by a significant rise of this energy sum. With respect to the algorithm, this means that the minimum of the sum curve must be automatically recognized from partial energy minus a negative trend 8 suitably selected according to the signal noise:       S    i    xe2x80x2    =                    ∑                  k          =          0                i            ⁢              xe2x80x83            ⁢              x        k        2              -          i      ⁢              xe2x80x83            ⁢      δ      
The trend may be represented e.g. as follows;   δ  =            S      N              α      ·      N      
SN is the energy at the last sample point N. An automatic iteration routine was implemented for the variable a value for adjustment to the signal quality.
The inventive method can be carried out in industrial practice for reliable and easy continuous monitoring of the state of a solidifying and/or hardening material by means of an inventive receptacle and an inventive ultrasound transmitter. The receptacle preferably comprises a U-shaped part (24) from a highly-dampening material and two rigid container walls (22,23), from a material which permits emitting plane waves, for mounting an ultrasound transmitter (3) and an ultrasound receiver (4), wherein the shaped part (24) and the container walls (22,23) delimit a receiving space for the material to be examined, characterized in that the U-shaped part (24) is pressed by means of connecting elements (25) engaging on the two container walls (22,23), between the two opposite container walls (22,23). The ultrasound transmitter preferably comprises means for generating the ultrasound pulses through acceleration of a sphere (8) to exert an impulse, having a large frequency content, onto the wall of a receptacle, characterized in that the means for accelerating the sphere (8) are formed by a compressed gas acting directly onto the sphere (8) or through an electric lifting magnet moved towards the sphere (8).
The shaped part of the receptacle acoustically decouples the container walls thereby providing at the same time secure sealing of the receiving space to prevent leakage of the material from the receiving space. The construction by means of the connecting elements facilitates assembly and disassembly of the receptacle into its individual parts for cleaning.
The ultrasound transmitter comprises means for accelerating a sphere which are formed by compressed gas or a movable lifting magnet. This permits reproducible ultrasound generation with simple means.
Although the invention is described with respect to the hardening of concrete, the inventive method or parts thereof is/are not limited to the examination of concrete but also applicable for other materials, composite materials, plastic materials etc.